Ascorbic acid in plants: biosynthesis and function.

Ascorbic acid (vitamin C) is an abundant component of plants. It reaches a concentration of over 20 mM in chloroplasts and occurs in all cell compartments, including the cell wall. It has proposed functions in photosynthesis as an enzyme cofactor (including synthesis of ethylene, gibberellins and anthocyanins) and in control of cell growth. A biosynthetic pathway via GDP-mannose, GDP-L-galactose, L-galactose, and L-galactono-1,4-lactone has been proposed only recently and is supported by molecular genetic evidence from the ascorbate-deficient vtc 1 mutant of Arabidopsis thaliana. Other pathways via uronic acids could provide minor sources of ascorbate. Ascorbate, at least in some species, is a precursor of tartrate and oxalate. It has a major role in photosynthesis, acting in the Mehler peroxidase reaction with ascorbate peroxidase to regulate the redox state of photosynthetic electron carriers and as a cofactor for violaxanthin de-epoxidase, an enzyme involved in xanthophyll cycle-mediated photoprotection. The hypersensitivity of some of the vtc mutants to ozone and UV-B radiation, the rapid response of ascorbate peroxidase expression to (photo)-oxidative stress, and the properties of transgenic plants with altered ascorbate peroxidase activity all support an important antioxidative role for ascorbate. In relation to cell growth, ascorbate is a cofactor for prolyl hydroxylase that posttranslationally hydroxylates proline residues in cell wall hydroxyproline-rich glycoproteins required for cell division and expansion. Additionally, high ascorbate oxidase activity in the cell wall is correlated with areas of rapid cell expansion. It remains to be determined if this is a causal relationship and, if so, what is the mechanism. Identification of the biosynthetic pathway now opens the way to manipulating ascorbate biosynthesis in plants, and, along with the vtc mutants, this should contribute to a deeper understanding of the proposed functions of this multifaceted molecule.